1. Field of the Invention
This invention relates to a method for applying a flux band on a sleeve to be affixed within a tube by a brazing procedure and, more particularly, for applying a melted-in-place, or ceramic, flux band on such a brazing sleeve which is hard and adherent, and can withstand surface abrasion, and to a brazing sleeve having such a ceramic flux band. The invention has special applicability for performing so-called "sleeving operations" in heat exchangers employed with nuclear reactor power plants, but is of general applicability to any requirement wherein a brazing sleeve must be inserted into and advanced coaxially through a tube for a considerable distance to a remote position at which it is to be brazed in place.
2. Desciption of the Prior Art
Numerous methods are known in the prior art for preparing a sleeve (i.e., a relatively smaller diameter tube of limited axial length) to be inserted into and advanced coaxially through a relatively larger diameter tube to a remote position at which it is to be brazed to the larger diameter tube, for repairing or sealing defects in the side wall of the larger diameter tube. Such techniques have particularly been developed for repairing the tubes of tube-type heat exchangers, or steam generators, employed with nuclear reactor power plants.
In tube-type heat exchangers, a primary recirculating fluid which is heated by the nuclear reactor flows through the tubes of the heat exchanger while a secondary fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids, converting the secondary fluid to steam. Occasionally, one of the tubes may develop a defect which can permit a leak to occur, allowing the fluids to mingle. This situation creates not only an ineffective heat exchanger, but also a serious problem of radioactive contamination of the secondary fluid. Therefore, when a leak occurs in a nuclear steam generator heat exchanger tube, the heat exchanger tube must either be plugged or repaired so that the primary fluid does not leak from the tube, and thus to prevent contamination of the secondary fluid.
There are several methods known in the art for repairing heat exchanger tubes; however, many of these methods are not applicable to repair of nuclear reactor heat exchanger tubes. For example, in a nuclear steam generator, the physical inaccessibility of defect regions of heat exchanger tubes and the radioactive nature of the environment surrounding the heat exchanger tubes present unique problems in their repair that do not normally exist in other heat exchangers. For these reasons, special methods have been developed for repairing heat exchanger tubes in nuclear steam generators. Typically, a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective heat exchanger tube is inserted into the latter and advanced coaxially therewithin to the region of the defect, and then is attached by brazing to the defective tube to bridge, or span, the defect region of the tube. This type of repair method is generally referred to as "sleeving". U.S. Pat. No. 4,448,343--Kochka et al., issued May 15, 1984 and assigned to Westinghouse Electric Corporation, assignee of the present invention, discloses a "sleeving" method for preparing, inserting and brazing sleeves in nuclear steam generator heat exchanger tubes, which method is capable of being performed where personnel access is limited and a where a leakproof joint is needed so that no leakage occurs from the tube in the defect region.
Previous sleeving development work has been concerned with obtaining a leakproof joint between the sleeve and the tube by brazing, arc welding, explosive welding, or other metallurgical bonding techniques. The need for cleanliness, close fittings, heat application, and atmospheric control in the environment of nuclear steam generators, however, presents problems in the implementation of these techniques which are not easily solvable.
A particular problem encountered with prior art methods arises out of inadequate adherence, to the sleeve, of the flux necessary for the brazing operation. Since alignment of the sleeve with the inside walls of the tube is not perfect, insertion of the sleeve into the tube can cause abrasion, and thus disruption and removal, of the pre-placed flux, resulting in defective brazed joints. The problem of course is magnified if the sleeve must be inserted a long distance within the tube to the intended brazing position. Prior art techniques, which attempt to solve the adherence problem by placing the flux underneath the braze alloy, or by cutting a recess in the outer circumference of the sleeve to hold the flux, have successfully protected the flux compound against abrasion; however, the brazed joints formed by these techniques frequently have been of unacceptable quality. The resultant joints thus may not be leakproof, creating a particularly critical problem in the environment of a heat exchanger tube for nuclear powered steam generators, for the reasons above set forth.